This invention relates to a light measuring device incorporated in a photographic camera, and more particularly, to a digital light measuring device of photocurrent integration type equipped with a microcomputer means as a controlling system.
Generally, the light measuring devices can be divided into two types, i.e., a photocurrent integration type and a photocurrent follow-up type, respectively, depending upon the difference in the functional characteristics therebetween.
More specifically, the photocurrent follow-up type mentioned above specifies the type which can successively output signals corresponding to momentary variation of the brightness of the object to be photographed, while on the contrary, in the photocurrent integration type, signal which corresponds to a total amount of light received by a photoelectric element incorporated in the device within a predetermined time-interval, is produced as an output.
Accordingly, in cases where pulse light such as light from electronic flash, etc. is to be measured, with the light measuring device of the photocurrent follow-up type employed for an exposure meter of a camera, there is such an inconvenience that the correct exposure may not be obtained due to follow-up nature inherent in the device mentioned above. Meanwhile, the light measuring device of photocurrent integration type wherein the light amount incident upon a photoelectric element is integrated for generating signal corresponding to the total sum of said light amount as output has the advantage that it becomes possible to measure the pulse light such as electronic flash light, etc., if employed for the exposure meter of a camera and the like.
Athough the photocurrent integration type is capable of measuring light such as the pulse light as described in the foregoing, when the photometric quantity produced as output signal is displayed in a digital mode, a number of additional components for accomplishing the above-mentioned purpose, for example, a counter, A-D converting part, and a decoder to be incorporated in a digital displaying device etc. are to be inevitably further included in the light measuring device. Consequently, if the light measuring device of the photocurrent integration type including the functions as described above is constituted by the conventional analog and digital circuit means, many circuit components are inevitably required and thereby, resulting in a high manufacturing cost and increased size of the light measuring device.
In order to overcome the disadvantages inherent in the known light measuring device as described above, there has conventionally been proposed the employment of the so-called microcomputer wherein control system, memory section, etc. are formed into one chip, with reduced power consumption, by which a light measuring device having a variety of functions may be achieved only by a small number of discrete circuit components, with consequent reduction in cost and size of the device. However, in the computerization of the light measuring device, problems as follows are further brought about due to the signal processing speed of the microcomputer, especially when a photo-trigger mode, in which an incidental light intensity of the electronic flash light is detected for measurement, is adapted for the device.
Generally, as is shown in FIG. 1, the shortest emitting duration of the automatically triggered electronic flash of the conventional type is in the vicinity of 40 .mu.sec. In spite of the fact mentioned above, it takes, for example, about 5 .mu.sec. for the measuring device to photoelectrically detect the incidental light intensity mentioned above, and further, 40 to 70 .mu.sec for the microcomputer incorporated in the device to process the detection output of the above-mentioned incidental light intensity after having received the instruction provided by a closed loop therein, and then, to successively produce a starting signal of integral action as an output therefrom. As is clear from the fact mentioned above, by the moment whereat the starting signal of integral action is output, the emission of the electronic flash will have already been completed, thus making the light measuring incapable.